Electrical alarm circuit



July 6, 1937. E. M. SQUlRE 2,085,773

ELECTRICAL ALARM CIRCUIT Filed Jan. 25, 1934 3 J 7 U? I F 1 4 61 15' (2 77260211 ZZJauz'r-e Patented July 6, 1937 ELECTRICAL ALARM CIRCUIT Edmund M. Squire, Brooklyn, N. Y., assignor to Signal Engineering & Manufacturing Company, New York, N. Y., a corporation of Massachusetts Application January 25, 1934, Serial No. 708,289

3 Claims.

The present invention relates to electrically controlled alarm circuits, and has for its object to provide an improved alarm circuit characterized by its ability to immediately detect and indicate by a suitable signal any tampering with the wires of the circuit, as by breaking, grounding or short circuiting the same, or by shunting a resistance across the wires.

The alarm circuit of the present invention has particular utility in connection with burglar alarm systems, which are frequently subjected to attempts by unauthorized persons to render them ineffective, the improved alarm circuit being so arranged as to immediately give warning of any tampering therewith and in a manner unknown to the person attempting to render the circuit inoperative. The above and other advantageous features of the invention will hereinafter more fully appear from the following description with reference to the accompanying drawing, in which: i

Fig. 1 is a wiring diagram of the circuit embodying the invention, in its normal condition.

Fig. 2 is a diagram similar to Fig. 1, showing the operation of the circuit to give an alarm upon the occurrence of any abnormal condition.

Like reference characters refer to like parts in the different figures.

Referring to the drawing, the circuit is shown for purposes of illustration in connection with a coin box alarm for telephone pay stations, although obviously the circuit may be employed for many other protective purposes, without substantial modification. As shown, the circuit is adapted to be energized from a suitable source of electrical energy represented by the primary winding 1 of a transformer, the secondary winding 2 of which is adapted to supply alternating current at a relatively low voltage to conductors 3 and 4 of the alarm circuit.

The conductor 3 includes in its circuit the winding 5 of a relay, the movable armature 6 of which is held out of engagement with a stationary contact 1 as long as the winding 5 remains energized. The armature 6 is connected to the conductor 3, while the stationary contact "I is connected to the other conductor 4 through a signal device 8 shown as being of the bell type, although any other suitable electrically operated signal of either the visual or audible type may obviously be employed. As long as the winding 5 of the relay remains energized, the signal 8 is maintained in an inoperative condition, with engagement of the contact 1 by the armature 6 causing immediate operation of the signal 8 upon deenergization of the winding 5.

The transformer, relay and signal 8 are shown disposed within a dot and dash line to indicate that this apparatus is grouped at a location in which it is desired to register the operation of the signal. For purposes of illustration, it may be assumed that the apparatus within the dot and dash line is located at a telephone exchange station from which the alarm circuit conductors 3 and 4 lead to a telephone pay station at a point remote from the exchange. That is to say, the length of the alarm circuit conductors 3 and 4 is indeterminate, and it is sufiicient for the purposes of the present invention to state that the conductors 3 and 4 terminate within the coin box 9 of the pay station, generally designated by the dot and dash lines.

In accordance with the present invention, the circuit represented by the alarm conductors 3 and 4- is terminated by a condenser It, and the capacity of the condenser i9 is such that a condition of resonance exists in the circuit, including the relay winding 5, the'conductors 3 and 4 and the condenser l0. Furthermore, the winding 5 of the relay is so designed that sufiicient current will flow through the winding to energize the same and hold the armature 6 away from the contact I only so long as the condition of resonance exists in the alarm circuit.

With the alarm circuit in the condition illustrated in Fig. 1, it is evident that any attempt to render the alarm circuit ineffective by a person seeking to rob the coin box will immediately result in operating the alarm signal 8. For example, should a resistance ll be bridged across the terminals of the condenser l0 as indicated in dotted lines in Fig. 2, the resonant condition of the alarm circuit will immediately be disturbed to the extent that the relay winding will. no longer be sufiiciently energized to attract its armature 6. Thereupon, the armature 6 willengage the stationary contact 7 to cause the signal 8 to be operated. In fact, the circuit is extremely sensitive to a relatively high bridging resistance. Similarly, short circuiting the terminals of the condenser IE3, or grounding of the conductor 3, will also disturb the resonant condition of the circuit and result in operation of the alarm signal 8. Furthermore, the circuit is adapted to work effectively over a wide resistance range in its connections, without necessity of special ad J'ustment thereof.

As an added protection against bodily removal of the coin box 9 without disturbing the condenser Ill, assuming this to be possible, there is included in the circuit of the conductor 3 a movable contact l2 which is maintained in engagement with a stationary contact [3 so long as the coin box 9 remains in its original position. The movable contact [2 is so arranged that any attempt to move the coin box 9 results in opening the contact, and the break in the conductor 3 immediately results in deenergization of the winding 5 and operation of the signal 8. Instead of series connected closed contacts I2 and I3, open shunt connected contacts l2 and I3 may be employed, as indicated in dotted lines Fig. 1, closure of which contacts serves to short circuit the condenser IO. i

From the foregoing, it is apparent that by the present invention there is provided an improved alarm circuit characterized by its ability to immediately detect and indicate, by operation of the signal 8, any tampering with the conductors 3 and 4 of the circuit for the purpose of rendering the circuit ineffective. In utilizing the invention as a protective circuit for a telephone coin box alarm, it is obvious that any attempt by persons seeking to rob the coin box to tamper with the circuit immediately results in sounding an alarm at the telephone exchange with which the pay station is connected. Under certain conditions it might be desirable to locate the alarm signal on the same premises with the telephone pay station to render more certain the apprehension of anyone seeking to rob the box, which result can be accomplished without any substantial modification of the arrangement shown and described.

I claim:

1. The combination of a device to be protected and alarm arrangements therefor comprising a supply circuit including a source of alternating current, an alarm circuit having appreciable resistance connected to said supply circuit and extending to said device to be protected, said alarm circuit having included therein a relay winding and a capacitance arranged in series and so adjusted that said alarm circuit will normally be a resonant circuit at the frequency of said alternating current source, a set of contacts on said device to be protected in shunt with said capacitance and adapted to be changed from their normal condition, means controlled by the operation of said contacts for making said alarm circuit not resonant at the frequency of said source, and means controlled by said relay winding when said alarm circuit is not resonant at the frequency of said source for operating an alarm.

2. The combination of a device to be protected and alarm arrangements therefor comprising a supply circuit including a source of alternating current, an alarm circuit having appreciable resistance connected to said supply circuit and extending to said device to be protected, means for completing said alarm circuit at said device to be protected through an element of capacitance, a set of normally open contacts connected to said device to be protected and adapted to be closed and connected to said alarm circuit whereby a short circuit may be closed about said element of capacitance, a relay included in said alarm circuit in series with said capacitance, and an alarm device controlled by said relay, the inductance of said relay and capacitance of said element being so adjusted that when said contacts are open said alarm circuit will be a series resonant circuit at i the frequency of said source of alternating current.

3. The combination of a device to be protected and an alarm arrangement therefor comprising a supply circuit including a source of alternating:

current, an alarm circuit having appreciable resistance connected to said supply circuit and including a pair of conductors extending from said source to said device to be protected, said alarm circuit having included therein a relay winding and a capacitance connected across the terminals of said conductors remote from said relay winding, in series therewith, and so adjusted that said alarm circuit will normally be in a resonant condition at the frequency of said alternating current source, with the bridging of said conductors between said capacitance and said relay winding serving to render said alarm circuit not resonant at the frequency of said source and means controlled by said relay winding when said alarm circuit is not resonant for operating an alarm.

EDMUND M. SQUIRE. 

